The invention is in the field of hollow composite tubes that are used in a variety of products, most notably golf club shafts, bicycle frame tube members, and fishing rods. These tubes are typically formed by rolling or winding several layers of resin pre-impregnated carbon fiber ("prepreg") or composite cloth around a mandrel, and heat-curing the wrapped mandrel. A design limitation inherent in this fabrication technique is that the resulting tube must be cylindrical or conical so that the mandrel is removable after the shell has cured. Although these shapes are the basic configurations that are needed, it is often desirable to embellish the basic shapes with a bulge, for forming a handgrip in a fishing rod, for example.
Such bulges have been made without having to abandon the mandrel-winding technique. A special mandrel is used, which has an internal air passageway entering from the end of the mandrel, and which branches out radially, opening to the mandrel surface at a region to be bulged. The mandrel is covered with a form-fitting expandable bladder and then is wound or rolled as usual with the composite or prepreg cloth. A split female mold is clam-shelled around the composite cloth and expandable bladder, thereby defining the shape of the bulge. Air is forced into the passageway at about two hundred pounds per square inch, entering and expanding the bladder and thus the prepreg cloth out against the inner surface of the mold. The prepreg or composite cloth is thus distorted so as to be formed into a larger diameter. The shell is subject to heat to cure the resin while this pressure is held, to produce a smooth bulge. Because air is the only internal forming material extending radially outside the base tube diameter, the mandrel is not obstructed on removal from the resulting hollow composite tube with an enlarged diameter section forming a smooth bulge.
This prior art process is widely used, but is slow and therefore expensive. It creates down time in an otherwise streamlined, rapid fabrication sequence. Air entrapment frequently creates undesirable voids in the composite laminate, which result in structurally inferior products. A corollary to the expense problem is, it creates a bias toward avoiding non-cylindrical or conical forms, leading to the production of less-useful tube shapes.
It is therefore an object of the present invention to provide a faster and much less expensive technique that also expands the capabilities of the producer to create more complex bulge configurations, having greater slopes and more intricate surface contours.
Another object of the present invention is to provide a method and apparatus to use thermoset composites rather than thermoplastics for composite shell fabrication techniques.